1. Field of the Invention
The present invention relates to an EL driving circuit, a control method and an apparatus more particularly to those having an electroluminescent element.
2. Description of the Prior Art
FIG. 6 is a circuit diagram of a conventional EL driving circuit. The EL driving circuit is arranged that an IC chip 40 applies voltage, which is supplied through a coil L and a resister R3 by a control signal Hon from a microcomputer to be not shown, to an EL element 50 to drive (Refer to U.S. Pat. No. 4,527,096).
In the IC chip 40, main circuits such as an oscillating circuit OSC, flip-flops FF1 and FF2 as a dividing circuit and a switched H-bridge circuit are provided. Terminals (PAD) of PIN 1 to 8 connecting to each circuit are also provided in the IC chip 40.
The oscillating circuit OSC is connected to a capacitor C3 through the PIN 7 and PIN8, and connected to the flip-flop FF1 in the IC chip 40. The oscillating circuit OSC generates a high frequency clock and outputs the clock to the flip-flop FF1.
The flip-flop FF1 outputs its output to the flip-flop FF2 and one of input terminals of an AND circuit 1 described below (hereinafter referred to as xe2x80x9cAND1xe2x80x9d).
The flip-flop FF2 generates a low frequency clock of which the clock outputted from the flip-flop FF1 is divided byl6. The flip-flop FF2 outputs its output, as two low frequency clocks in reversal phase each other to output them to one of input terminals of an AND circuit 2 described below (hereinafter referred to as xe2x80x9cAND2xe2x80x9d) and one of input terminals of an AND circuit 3 described below (hereinafter referred to as xe2x80x9cAND3xe2x80x9d) respectively.
The switched H-bridge circuit is constituted by main elements such as ANDs 1, 2 and 3, transistors Tr1, 2 and 3, diodes 1 to 4 and thyristors SCR1 and 2, and those elements are connected as described below.
Each one of input terminals of the ANDs 1, 2 and 3 is connected to the PIN1 respectively. The control signal Hon from the microcomputer not to be shown is connected to the PIN1
The PIN6 is a power supply terminal Vdd of the IC chip 40 and connected to a positive electrode vdd of a power supply such as a battery and the like. The PIN2 is a power supply terminal GND of the IC chip 40 and connected to a negative electrode of the power supply such as the battery and the like. The PIN2 and PIN6 are connected to Vdd and GND in an inner circuit of the IC to supply electric power respectively. A capacitor C2 is located outside the IC and connected to the PIN2 and PIN6 to operate as an electrolytic compensating capacitor.
Wiring between the other input terminals of ANDs 1,2, and 3 and the PIN1 is connected to a pull-down resister R1 connected to the GND. An output of the AND1 is connected to a base of the transistor Tr1 through a capacitor C1 and a resistor R2 which are connected in parallel. An output of the AND2 is connected to a base of the transistor Tr2, and an output of the AND3 is connected to a base of the transistor Tr3.
A collector of the transistor Tr1 is connected to the PIN3. Between the PIN3 and the positive electrode Vdd of the power supply, the voltage set-up coil outside the IC is connected, strictly the resister R3 and the coil L are connected in series. Anodes of the diodes D1 and D3 are connected to wiring between the transistor Tr1 and the PIN3.
A cathode of the diode D1 is connected to an anode of the thyristor SCR1. An gate of the thyristor SCR1 is connected to a collector of the transistor Tr2. A cathode of the thyristor SCR1 is connected to the PIN4 and an anode of the diode D2. A cathode of the diode D2, like the gate of the thyristor SCR1, is connected to the collector of the transistor Tr2.
A cathode of the diode D3 is connected to an anode of the thyristor SCR2. A gate of the thyristor SCR2 is connected to a collector side of the transistor Tr3. A cathode of the thyristor SCR2 is connected to the PIN5 and an anode of the diode D4. A cathode of the diode D4, like the gate of the thyristor SCR2, is connected to the collector of the transistor Tr3.
Emitters of the transistors Tr1, Tr2 and Tr3 are connected to the GND respectively. The PIN4 and PIN5 are connected to a capacitor C4 which is an equivalent circuit of the EL element 50.
Operation of above described configuration of the EL driving circuit is explained below. When the EL turns on, the microcomputer not to be shown switches the signal Hon from a low state to a high state. The signal is inputted to one of inputs of the AND1, AND2 and AND3 through the PIN1 respectively.
At this point, when the higher frequency clock signal generated by the oscillating circuit OSC is inputted to the other input terminal of the AND1 through the flip-flop FF1, the transistor Tr1 performs switching operation responsive to the clock. The transistor Tr1 applies pulsating voltage to the diodes D1 and D3 through the coil L and the resister R3 by the switching operation.
On the other hand, inputting the lower frequency clock from the flip-flop FF2 causes the AND2 and AND3 to output the high state alternately, which causes the transistors Tr2 and Tr3 to perform the switching operation alternately.
When output from the AND2 exists in the base of the transistor Tr2, the transistor Tr2 turns on, the thyristor SCR1 turns off, and the PIN4 becomes almost equivalent electric potential as the GND through the diode D2. At this point, the transistor Tr3 is turned off. Electric charge is being accumulated in the capacitor C4 through the diode D3 and the thyristor SCR2 by the pulsating voltage. In this way, voltage generating in the capacitor C4 is gradually approaching a saturation state.
When output from the flip-flop FF2 is inversed, the transistor Tr2 turns off and the transistor Tr3 turns on. At this point, the thyristor SCR2 turns off, and the PIN5 becomes almost equivalent electric potential as the GND through the diode D4. The electric charge is being accumulated in the capacitor C4 through the diode Dl and the thyristor SCR1 by the pulsating voltage. The voltage generating in the capacitor C4 is gradually approaching a saturation state.
As described above, polarity of voltage applied to the capacitor C4 is switched alternately and the voltage is continuously supplied to the capacitor C4, which permits high voltage to be supplied to the EL element 50.
FIG. 7 is a timing chart of an EL driving circuit. The timing chart shows timing of relationship between voltage and time. When the signal Hon inputted from the PIN1 to the IC chip 40 is turned on, the timing chart shows appearance that, in case that the clock (CLOCK) outputted from the oscillating circuit OSC to the flip-flop FF1 is supplied to the switched H-bridge circuit, responsive to the clock from the flip-flop FF2 not to be shown in FIG. 7, voltage (EL1) of the PIN5 and voltage (EL2) of the PIN4 are alternately set-upped to be supplied to the capacitor C4.
However, in the above described conventional EL driving circuit, when the EL element is turned off, the microcomputer switches the signal Hon from a high state to a low state at the timing (duration X shown in FIG. 7) when the polarity of the voltage applied to the capacitor of the EL device is reversed, which causes the transistor (Tr1) to be turned off, thus creating the counter electromotive force Vcoil1 in the PIN3. A problem is that, when the Vcoil1 exceeds a rating Vces of the transistor (Tr1), the transistor (Tr1) to be turned off turns on, which causes noise such as power supply fluctuation and the like to be created.
Particularly, the noise is created from the coil as electromagnetic wave, the microcomputer is sometimes reset by error due to the electromagnetic wave, consequently there exerts a problem that the noise causes operation of the microcomputer to be unstable. For example, there exists a problem that, in case where the microcomputer includes time function, time is reset by the noise to become 00:00.
In the meantime, it was found by the present inventors that, by analyzing the conventional EL driving circuit, difference in the Vces of the transistor (Tr1) due to the unevenness of the IC chip brought about by mass production causes the above described creation of the noise by operation of the transistor (Tr1).
Generally, the Vces is a characteristic of collector to emitter maximum voltage in a transistor and represented by a characteristic curve between collector current Ic and collector-emitter voltage Vce.
That is to say, it is found that, in the conventional EL driving circuit, when the rating Vces of the transistor (Tr1) is lower than a design value, the counter electromotive force created in the coil at the timing of turning off the EL element as described above exceeds a voltage of the rating Vces of the transistor (Tr1), which causes the transistor (Tr1) to be turned on by error due to the counter electromotive force to create the noise from the coil (L).
FIG. 8 is a timing chart in case that the rating Vces of the transistor (Tr1) is higher than the design value. FIG. 9 is a timing chart in case that the rating Vces of the transistor (Tr1) is lower than the design value.
As shown in FIG. 8, in case that the rating Vces is higher than the design value, when the signal Hon which directs turning off the EL element falls, a waveform of the counter electromotive force generated from the coil becomes a single-angled form.
On the other hand, in case that the rating Vces is lower than the design value, when the signal Hon which directs turning off the EL element falls, the counter electromotive force generated from the coil rises from 0V, and avalanche breakdown occurs when the counter electromotive force exceeds the rating Vces. As a result, as shown in FIG. 9, a waveform of the PIN3 becomes a plurality of the waveforms of saw-tooth and angled forms. The saw-tooth wave creates the noise, which makes the operation of the microcomputer unstable. Spike noise to be seen in the signal Hon is created through a measuring equipment by noise in PIN3.
In accordance with the foregoing, it is a purpose of the invention to provide an EL driving circuit suppressing the counter electromotive force of the coil generated in case of turning off the EL element to eliminate creation of the noise from the coil by the avalanche breakdown, and thereby preventing the error of the microcomputer due to the noise.
In order to accomplish the purpose described above, an EL driving circuit according to the invention comprises an EL element, a voltage set-up coil which generates pulsating voltage for turning on the EL element, an oscillating circuit for generating a reference signal, a dividing circuit for dividing a clock generated by the oscillating circuit into at least two kinds of frequency clocks, a first switching circuit for switching current through the voltage set-up coil by the higher frequency clock of the two kinds of frequency clocks divided by the dividing circuit, a second switching circuit for switching polarity of voltage applied to the EL element by the lower frequency clock of the two kinds of frequency clocks divided by the dividing circuit, a control circuit for switching connection between the dividing circuit and the first switching circuit, and connection between the dividing circuit and the second switching circuit at the same time by a control signal which turns on and off the EL element, and a suppressing circuit for suppressing counter electromotive force created in case of turning off the EL element lower than the withstanding voltage of the first switching circuit.
The suppressing circuit enables the counter electromotive force generated in case of turning off the EL element to be lower than the withstanding voltage of the first switching circuit. The EL element may be made of either inorganic EL materials or organic EL materials.
Consequently even in the same period of time as the conventional case that voltage generated in case of turning off the EL element is higher than the rating Vces, the voltage of the coil L can be suppressed below the rating Vces by the suppressing circuit. As a result, error of the microcomputer can be prevented.
Another EL driving circuit is that the suppressing circuit includes a first delay circuit constituted by a combination of a resistor and a capacitor, and the first delay circuit is arranged at a forestep of the control circuit, in case that the control signal turning off the EL element is inputted to the first delay circuit, input voltage of the first delay circuit is switched from a high state to an open state.
This causes a signal which controls turning on and off the EL element to be relaxed by the first delay circuit to be outputted to the control circuit.
Consequently, even in the same period of time as the conventional case that voltage generated in case of turning off the EL element is higher than the rating Vces, the voltage of the coil L can be suppressed below the rating Vces by the delay circuit. As a result, error of the microcomputer can be prevented.
Another EL driving circuit is that the suppressing circuit includes a resistor connected in parallel to the voltage set-up coil and suppresses the counter electromotive force in case of turning off the EL element.
Accordingly, in case of turning off the EL element, because pulsating voltage generated in case of turning off the switching transistor is consumed as Joule heat by the resistor, electromagnetic wave oscillated from the voltage set-up coil can be reduced. As a result, the error of the microcomputer due to the electromagnetic wave generated in case of turning off the switching transistor can be prevented.
Another EL driving circuit is that the suppressing circuit includes a second delay circuit constituted by a combination of a resistor, a capacitor and a third switching circuit, and in the second delay circuit, the third switching circuit turns on in case of turning off the EL element.
Accordingly, fluctuation of voltage supplied to a base of the first switching transistor can be made gentle by a resistor and a capacitor, which causes change of current through the coil L to become small even in the same period of time as the conventional case that the voltage generated in case of turning off the EL element is higher than the rating Vces. This permits voltage of the coil L to be suppressed below the rating Vces. As a result, error of the microcomputer can be prevented.
It is desirable that a third switching element for switching the capacitor is provided, the switching element is turned on in case of turning off the EL element and electric charge is charged by voltage applied to a gate of a third switching transistor.
Another EL driving circuit is that a signal generating circuit for generating a signal which controls turning on and of f the EL element is provided, and the signal generating circuit generates the control signal which turns on and off the EL element. And the invention is also that a control method comprises steps of generating a control signal which changes a high state to an open state for turning off the EL element, inputting the control signal to a delay circuit and decreasing the voltage of the delay circuit from the high state according to a time constant, generating an irregular pulse by the control circuit when the decreasing voltage of the delay circuit reaches to certain a voltage to threshold voltage of the control circuit, and releasing energy stored in a voltage set-up coil by a first switching element which turns on or off current through the voltage set-up coil responsive to the irregular pulse.
Accordingly, when the control signal is inputted to the delay circuit, the voltage of the delay circuit decreases from the high state according to the time constant and reaches a certain voltage for the threshold voltage of the control circuit, then the control circuit generates the irregular pulse and the first switching element turns on or off current flowing through the voltage set-up coil, which permits energy stored in the voltage set-up coil to release.
At this point, the time constant of the delay circuit is not lower than 10 microseconds and not more than 1 millisecond.
Even in the same period of time as the conventional case that voltage generated in case of turning off the EL device is higher than the rating Vces, the voltage of the coil L can be suppressed below the rating Vces because energy stored in the voltage set-up coil is released. As a result, error of the microcomputer can be prevented.
The invention is also that a control method comprises steps of generating a control signal which changes a high state to an open state for turning off the EL element, starting delay operation by turning on a switch of the delay circuit when the control signal is inputted to the delay circuit, outputting a low signal to the delay circuit by the control circuit when the control signal is inputted to the control circuit, outputting a signal decreased according to the time constant of the delay circuit when the low signal is inputted to the delay circuit, and decreasing current through the voltage set-up coil responsive to the signal by the first switching circuit when the signal is inputted to the first switching circuit.
Accordingly, fluctuation of voltage supplied to a base of the switching transistor can be made gentle by a resistor and a capacitor, which causes change of current through the coil L to become small even in the same period of time as the conventional case that voltage generated in case of turning off the EL element is higher than the rating Vces. This permits voltage of the coil L to be suppressed below the rating Vces. As a result, error of the microcomputer can be prevented.
The invention also relates to an electronic apparatus that comprises a power supply and the EL driving circuit driven by electric power from the power supply. The electronic apparatus includes a timepiece, a display, an electronic calculator, a mobile phone, an audio hardware, a computer of handheld type, a PDA (Personal Digital Assistance), and so on.